The invention relates to a metal halogen vapour lamp provided with a heat reflecting layer, a discharge tube made of a light permeable material with a high melting point, preferably quartz; wherein an arc discharge takes place in the discharge tube between high melting point electrodes therein. The discharge tube contains a metal of high vapor pressure at operational temperature, a noble gas and, as additives, at least one metal and a halogen which forms a halogen compound with a said metal.
At each end of the discharge tube of the lamp according to the invention, which tube is a material of high melting point, preferably quartz, there is a heat reflecting layer which is different from and has advantageous properties over know heat reflecting layers.
U.S. Pat. No. 3,234,421 discloses metal halogen vapour lamps widely used in the industry and for outdoor illumination. The appearance of said lamps is similar to the traditional, high pressure mercury vapour lamps provided with a quartz discharge tube. The discharge tubes of these commercially available lamps contain, in certain cases, beside mercury sodium iodide, thallium iodide and indium iodide, and in other cases they contain iodiodes of rare earth metals, cesium iodide and thallium iodide, however, several other versions are available.
During normal operation, the two ends of the discharge tube of the known lamps are the coldest points thereof. If necessary precautions are not carried out to increase the temperature of the ends of the discharge tube, certain metal halide compounds quickly condense at each end of the discharge tube behind the electrodes. This decreases the efficiency of the lamp and renders the lamps unserviceable, because some metal halogen compounds having an important role in discharge get separated from the discharge gas mixture. In order to prevent separation, generally heat and light reflecting layers are applied onto the ends of the discharge tube with the purpose to increase the temperature at the ends of the discharge tube.
One version of the so-called heat reflecting coating widely used for said purpose is described in U.S. Pat. No. 3,374,377. According to the disclosure of said patent, the heat reflecting layer consists essentially of ZrO.sub.2. The ZrO.sub.2 coating displays proper heat insulating features, since it neither blackens nor emits detrimental gases into the space between the discharge tube and the outer bulb. However, no satisfactory means of applying the heat reflecting layer was developed. The methods used result in a brittle coating which tends to peel off, does not stick well and, due to the impacts occuring during transport and handling, easily scales off.
Graphite layers are applied by the widely used method which is based on the perception that graphite absorbs a larger quantity of heat energy on the discharge side, than the uncoated quartz. This layer can be easily prepared but it yields satisfactory results with certain additives only (mostly NaI, TlI, InI).